Plastic lenses are light in weight and excellent in toughness, and are easy to dye. Performance particularly required for plastic lenses includes low specific gravity, high transparency and low yellowness index, high refractive index and high Abbe's number as optical performance, high heat resistance, high strength and so on. High refractive index allows a reduction in lens thickness, while high Abbe's number reduces chromatic aberration in lenses.
In recent years, there have been many reports of using organic compounds containing a sulfur atom(s) in an attempt to achieve high refractive index and high Abbe's number. Among them, a polyepisulfide compound containing sulfur atoms is known to achieve a good balance between refractive index and Abbe's number (Patent Document 1). Moreover, such a polyepisulfide compound is reactive with various compounds and therefore compositions thereof containing various compounds for the purpose of improving physical properties (Patent Documents 2 to 5).